As a next-generation communication system of the W-CDMA (Wideband Code Division Multiple Access) system, HSDPA (High Speed Downlink Packet Access) system, HSUPA (High Speed Uplink Packet Access) system and the like, namely an LTE (Long Term Evolution) system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of the W-CDMA. In the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied as prospective candidate schemes to be applied to the downlink communications system and the uplink communications system, respectively (see, for example, Non Patent Document 1).
The OFDM scheme is a multi-carrier transmission scheme in which a frequency band is divided into plural sub-carriers having narrower frequency bands, and data are mapped onto the sub-carriers. By closely and orthogonally arranging the sub-carriers (partially overlapping, though) along the frequency axis, the achievement of faster transmission and further improvement of the efficiency of using the frequency are expected.
The SC-FDMA scheme is a single carrier transmission scheme in which a frequency band is divided with respect to user equipment (hereinafter may be referred to as a user equipment (UE) terminal or a mobile station) in a manner so that different frequencies can be separately used (allocated) among plural terminals (user equipment (UE) terminals); and as a result, interference between the terminals may be easily and effectively reduced. Further, preferably, in the SC-FDMA scheme, a range of transmission power fluctuation may be made smaller; therefore, lower energy consumption of terminals may be achieved and a wider coverage area may also be obtained.
In both uplink and downlink of the LTE system (scheme), communications can be performed by sharing one or more physical channels among plural user equipment (UE) terminals. The channel shared plural user equipment (UE) terminals may generally be called a shared channel. In the LTE system, the channel in uplink is called a Physical Uplink Shared Channel (PUSCH) and the channel in downlink is called a Physical Downlink Shared Channel (PDSCH). Further, as the logical channel, the shared channel in uplink is called an Uplink Shared Channel (UL-SCH) and the shared channel in downlink is called a Downlink Shared Channel (DL-SCH).
Further, in the communication system using the shared channels, it is required to perform Signaling to report which shared channel is to be allocated to which user equipment (UE) terminal with respect to each sub-frame (having 1 ms in the LTE scheme). To perform the Signaling, a control channel is generally used. In the LTE system, the control channel may be called a Physical Downlink Control Channel (PDCCH) or a Downlink L1/L2 Control Channel (DL-L1/L2 Control Channel). The Physical Downlink Control Channel (PDCCH) includes information items indicating such as Downlink Scheduling Information, Acknowledgement Information (ACK/NACK), Uplink Scheduling Grant, an Overload Indicator, a Transmission Power Control Command Bit and the like (see, for example, Non Patent Document 2). The Acknowledgement Information (ACK/NACK) may also be called a Physical Hybrid ARQ Indicator Channel (PHICH). The Physical Hybrid ARQ Indicator Channel (PHICH) may be defined as a separate physical channel similar to the Physical Downlink Control Channel (PDCCH).
Further, the sub-frame may also be called a TTI (Transmission Time Interval).
The Downlink Scheduling Information and the Uplink Scheduling Grant correspond to the information to be used to perform the Signaling to report which shared channel is to be allocated to which user equipment (UE) terminal. The Downlink Scheduling Information includes the information items of the shared channel in downlink, for example, the information items indicating allocation information of Resource Blocks for downlink, ID of the user equipment (UE) terminal, the number of streams, information of a Precoding Vector, data size, modulation scheme, information of an HARQ (Hybrid Automatic Repeat reQuest) and the like. On the other hand, the Uplink Scheduling Grant includes the information items of the shared channel in uplink, for example, the information items indicating allocation information of Resource Blocks for uplink, ID of the user equipment (UE) terminal, data size, modulation scheme, transmission power information in uplink, information of Demodulation Reference Signal in Uplink MIMO and the like.
In the meantime, as an HARQ scheme in uplink of the LTE system, a Synchronous HARQ scheme is proposed to be applied (see Non Patent Document 3) to the LTE system. When the Synchronous HARQ scheme is applied, the Uplink Scheduling Grant is transmitted from the base station apparatus to the user equipment (UE) terminal with respect to initial transmission of an uplink shared channel; however, the Uplink Scheduling Grant is not transmitted with respect to retransmission of the uplink shared channel. In this case, when the user equipment (UE) terminal is required to retransmit the uplink shared channel, the user equipment (UE) terminal retransmits the uplink shared channel at a predetermined timing. On the other hand, there is another proposal that the Uplink Scheduling Grant be arranged to be transmitted with respect to the retransmission of the uplink shared channel on an as-needed basis (see Non Patent Document 4). In this case as well, the user equipment (UE) terminal retransmits the uplink shared channel at the predetermined timing.
Further, when the HARQ is applied in uplink of the (LTE) system, the Acknowledgement Information (ACK/NACK) with respect to the uplink shared channel is transmitted from the base station apparatus to the user equipment (UE) terminal; and based on the received Acknowledgement Information (ACK/NACK), the user equipment (UE) terminal determines whether the user equipment (UE) terminal should retransmit the uplink shared channel and retransmits the uplink shared channel based on a result of the determination. The Acknowledgement Information (ACK/NACK) herein corresponds to the Acknowledgement Information included in the Physical Downlink Control Channel (PDCCH), as described above.    Non Patent Document 1: 3GPP TR 25.814 (V7.0.0), “Physical layer Aspects for Evolved UTRA,” June 2006    Non Patent Document 2: 3GPP R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding    Non Patent Document 3: 3GPP TR 36.300 (V0.3.1), “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2, 9.1,” November 2006    Non Patent Document 4: 3GPP R1-070060, “Resource fragmentation in LTE uplink,” January 2007